In an optical device, various structures for reducing light reflections at an end of an optical waveguide have been proposed. The low-reflection structures can be generally divided into two types.
One type is applied to a case in which an output end of the optical waveguide matches an end face of the optical device. In this case, the reflected light at the output end is reduced by applying a non-reflective coating to the end face. Another method for obtaining the low-reflection structure by adjusting the arrangement of the optical waveguide is also known. In this case, the reflected light at the end face can be reduced by arranging the optical waveguide obliquely with respect to the end face, not vertical to the end face.
The other one type is applied to a case in which the end of the optical waveguide is located inside the optical device. In this case, by tapering the end of the optical waveguide, the refractive index of a clad and the equivalent refractive index of the optical waveguide can be brought closer together. It is therefore possible to suppress Fresnel reflections at the end and to reduce the reflected light. This method is suitable for a waveguide such as a silica optical waveguide in which the refractive index difference between a core (an optical waveguide) and the clad is small.
Further, it has been proposed to provide a terminator to terminate the optical waveguide inside the optical device. A method of connecting, in an OTDR (Optical Time Domain Reflectometer) measurement, an optical fiber having a terminator to an end of an optical fiber to be measured and decreasing the changes in the intensities of a return light measured at the end of the optical fiber to be measured is, for example, proposed (Patent Literature 1). According to this method, it is possible to accurately detect breaking, bending or the like near the end of the optical fiber to be measured.